I have been asked this many times on the board so I thought to publish these steps in its own thread for others.
I use garageband on a mac. It is super easy on the front end. Download a midi song from a free midi site. Drag the midi file onto garage band. You will see it import all instrument tracks. Delete all tracks except bass and drum track. Change the Bass Key by selecting all notes in that track and dragging them ALL up to the E3 and above Range so BB can see the bass notes. (This is cool because when you find the ROOT note when you Drag up you will see the Letter of the root note change as you drag and then just let it go when you get it above E3. i.e. for a song in C major- grab the C1 note and Drag to C4- make sure ALL notes are highlighted so they all move with the root note.) Next Select/Highlight the Bass and drum track together and then use the JOIN command in the edit menu to merge the tracks into one. Then highlight that merged track and select “add to loop library” in the edit menu and name the loop something you will remember so you can find the AIF file it creates later.
Unfortunately GB does not allow you to export as a .mid file. You need to convert the .aif file to .mid. The backend steps to find the AIF file GB creates and to convert it to Midi is more complicated with a few specific steps below.
Download the free app GB2MIDI (google it). This app literally converts an AIF file to a midi file that you can then import right into BeatBuddy. It places the newly created midi file in the same directory as the AIF file- right next to it with the same name.
Read the GB2MIDI app instructions carefully on how to find that AIF file and its directory. This is probably the hardest part. Garageband stores it in a directory that is hard to find. But once you find that directory make a shortcut for that directory and place on your desktop -and from then on it is easy peasy.
Note- I am working with Persist who a verified an issue with the new versions of BB manager that when you export a pre-exisiting Beat Buddy midi file (from the developer or others who have posted songs in the resource area) from the new manager it messes the midi file up and makes it un-editable in some DAWS and Garageband is one of those. He has reported this verified issue to the developer.
But for stock MIDI songs right off the web it has allowed me to create many One Press songs with bass and drums.
P.s.- you can easily chop songs up in GB too.